An ink jet head has nozzles, ink chambers, an ink supply system, an ink tank, and transducers. By transmitting displacement/pressure generated by the transducers to the ink chambers, ink particles are ejected from the nozzles, therefore characters or images are recorded on a recording medium such as paper.
In a well-known form, a thin-plate-shaped piezoelectric element having the whole of one surface thereof bonded to the outer wall of an ink chamber is used as each transducer. A pulse-like voltage is applied to the piezoelectric element, thus bending the composite plate comprising the piezoelectric element and the outer wall of the ink chamber, and the displacement/pressure generated through the bending is transmitted to the inside of the ink chamber via the outer wall of the ink chamber.
A sectioned perspective view of a conventional ink jet head using such piezoelectric elements is shown in FIG. 37. As shown in FIG. 37, the head is constituted from piezoelectric bodies 90, individual electrodes 91 formed on the piezoelectric bodies 90, a nozzle plate 93 in which are provided nozzles 92, ink chamber walls 95 made of a metal or a resin that, along with the nozzle plate 93, form ink chambers 94 corresponding respectively to the nozzles 92, and a diaphragm 96.
The nozzles 92 and the diaphragms 96 each correspond to the ink chambers 94; the periphery of the diaphragm 96 is connected strongly to the periphery of the corresponding ink chamber 94, and each piezoelectric body 90 deforms the corresponding diaphragm 96 as shown by the dashed lines in the drawing.
Regarding the application of voltages to the piezoelectric bodies 90, the diaphragm 96 is taken as a common electrode and is earthed, and electrical signals from a printing apparatus main body are applied separately to the individual electrodes 91 via a printed circuit board, not shown.
Regarding the formation of the piezoelectric bodies on the head, the most common method has been to bond on plate-shaped piezoelectric bodies in positions corresponding to the ink chambers 94, or to bond a piezoelectric body that spans a plurality of ink chambers in a position corresponding to the ink chambers, and then divide this piezoelectric body into individual piezoelectric bodies by cutting away or the like. With such head formation, in the case of forming thin piezoelectric bodies (<50 μm), there have been problems in that fluctuations in the thickness of the adhesive result in fluctuations in the characteristics, and hence the head driving characteristics deteriorate, and moreover bonding may not be possible (splitting may occur during bonding).
In contrast with the above method, it has been proposed to form a head with thin-film piezoelectric bodies by forming actuator parts comprising the piezoelectric bodies on a substrate, forming the pressure chambers, and then removing the substrate from the part that contributes to ink ejection.
With a bimorph type ink jet head using thin-film piezoelectric bodies as described above, the characteristics of the piezoelectric elements can be improved even though they are thin films, and in particular a high-density multi-nozzle head can be realized. Moreover, with this thin-film head, to obtain the very best actuator performance, it is necessary to carry out optimization of the diaphragm (thickness, hardness, electrical characteristics).
Moreover, to optimize the diaphragm, making the diaphragm be a multi-layer structure of an electrode and a diaphragm has been proposed from hitherto (for example, Japanese Patent Application Laid-open No. H7-81070). However, with this conventional multi-layer constitution proposal, the functioning of the electrode is improved, but consideration has not been given to optimization as a diaphragm for thin-film piezoelectric elements.
That is, to apply a multi-layer diaphragm to thin-film piezoelectric elements, when making the piezoelectric elements be thin films, it is necessary to also make the diaphragm (including the electrode) thin. In this case, with the piezoelectric elements being made to be thin films, the driving force/generated force of the piezoelectric elements becomes low, and to obtain the maximum volume change/generated pressure from the pressure chambers in this case, it is necessary to optimize the (mechanical) characteristics of the diaphragm, which inhibits the expansion and contraction of the elements.
Separate to this, considering optimization as an electrode, it is necessary to aim for optimization in both mechanical and electrical respects.
With the conventional proposal, these two are functionally separated, with the diaphragm having a multi-layer structure in which the electrical part is an electrode layer and the mechanical part is a rigid layer, but no consideration is given to making the diaphragm a thin film, and hence it is difficult to realize a diaphragm that is optimal for thin-film piezoelectric elements.